Systems and methods for storing spent nuclear fuel

ABSTRACT

Systems and methods for storing spent nuclear fuel below grade that afford adequate ventilation of the spent fuel storage cavity. In one aspect, the invention is a system comprising: a shell forming a cavity for receiving a canister of spent nuclear fuel, at least a portion of the shell positioned below grade; and at least one inlet ventilation duct extending from an above grade inlet to a below grade outlet at or near a bottom of the cavity; the inlet ventilation duct connected to the shell so that the cavity is hermetically sealed to ingress of below grade fluids. In another aspect, the invention is a method comprising: providing a below grade hole; providing a system comprising a shell forming a cavity for receiving a canister of spent nuclear fuel, at least a portion of the shell positioned below grade, and at least one inlet ventilation duct extending from an inlet to an outlet at or near a bottom of the cavity, the inlet ventilation duct connected to the shell; positioning the apparatus in the hole so that the inlet of the inlet ventilation duct is above grade and the outlet of the inlet ventilation duct into the cavity is below grade; filling the hole with engineered fill; and lowering a spent fuel canister into the cavity.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 13/323,743, filed Dec. 12, 2011, which is a continuation ofU.S. patent application Ser. No. 11/054,898, filed Feb. 10, 2005, whichis a continuation-in-part of U.S. patent application Ser. No.10/803,620, filed Mar. 18, 2004, and granted as U.S. Pat. No. 7,068,748,the entireties of which are hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention related generally to the field of storing spentnuclear fuel, and specifically to systems and methods for storing spentnuclear fuel in ventilated vertical modules.

BACKGROUND OF THE INVENTION

In the operation of nuclear reactors, it is customary to remove fuelassemblies after their energy has been depleted down to a predeterminedlevel. Upon removal, this spent nuclear fuel is still highly radioactiveand produces considerable heat, requiring that great care be taken inits packaging, transporting, and storing. In order to protect theenvironment from radiation exposure, spent nuclear fuel is first placedin a canister. The loaded canister is then transported and stored inlarge cylindrical containers called casks. A transfer cask is used totransport spent nuclear fuel from location to location while a storagecask is used to store spent nuclear fuel for a determined period oftime.

In a typical nuclear power plant, an open empty canister is first placedin an open transfer cask. The transfer cask and empty canister are thensubmerged in a pool of water. Spent nuclear fuel is loaded into thecanister while the canister and transfer cask remain submerged in thepool of water. Once fully loaded with spent nuclear fuel, a lidtypically placed atop the canister while in the pool. The transfer caskand canister are then removed from the pool of water, the lid of thecanister is welded thereon and a lid is installed on the transfer cask.The canister is then properly dewatered and filled with inert gas. Thetransfer cask (which is holding the loaded canister) is then transportedto a location where a storage cask is located. The loaded canister isthen transferred from the transfer cask to the storage cask for longterm storage. During transfer from the transfer cask to the storagecask, it is imperative that the loaded canister is not exposed to theenvironment.

One type of storage cask is a ventilated vertical overpack “VVO”). A VVOis a massive structure made principally from steel and concrete and isused to store a canister loaded with spent nuclear fuel. VVOs standabove ground and are typically cylindrical in shape and extremely heavy,weighing over 150 tons and often having a height greater than 16 feet.VVOs typically have a flat bottom, a cylindrical body having a cavity toreceive a canister of spent nuclear fuel, and a removable top lid.

In using a VVO to store spent nuclear fuel, a canister loaded with spentnuclear fuel is placed in the cavity of the cylindrical body of the VVO.Because the spent nuclear fuel is still producing a considerable amountof heat when it is placed in the VVO for storage, it is necessary thatthis heat energy have a means to escape from the VVO cavity. This heatenergy is removed from the outside surface of the canister byventilating, the VVO cavity. In ventilating the VVO cavity, cool airenters the VVO chamber through bottom ventilation ducts, flows upwardpast the loaded canister, and exits the VVO at an elevated temperaturethrough top ventilation ducts. The bottom and top ventilation ducts ofexisting VVOs are located circumferentially near the bottom and top ofthe VVOs cylindrical body respectively, as illustrated in FIG. 1.

While it is necessary that the VVO cavity be vented so that heat canescape from the canister, it is also imperative that the VVO provideadequate radiation shielding and that the spent nuclear fuel not bedirectly exposed to the external environment. The inlet duct locatednear the bottom of the overpack is a particularly vulnerable source ofradiation exposure to security and surveillance personnel who, in orderto monitor the loaded overpacks, must place themselves in close vicinityof the ducts for short durations.

Additionally, when a canister loaded with spent nuclear fuel istransferred from a transfer cask to a storage VVO, the transfer cask isstacked atop the storage VVO so that the canister can be lowered intothe storage VVO's cavity. Most casks are very large structures and canweigh up to 250,000 lbs. and have a height of 16 ft. or more. Stacking atransfer cask atop a storage VVO/cask requires a lot of space, a largeoverhead crane, and possibly a restraint system for stabilization.Often, such space is not available inside a nuclear power plant.Finally, above ground storage VVOs stand at least 16 feet above around,thus, presenting a sizable target of attack to a terrorist.

FIG. 1 illustrates a traditional prior art VVO 2. Prior art VVO 2comprises flat bottom 17, cylindrical body 12, and lid 14. Lid 14 issecured to cylindrical body 12 by bolts 18. Bolts 18 serve to restrainseparation of lid 13 from body 12 if prior art VVO 2 were to tip over.Cylindrical body 12 has top ventilation ducts 15 and bottom ventilationducts 16. Top ventilation ducts 15 are located at or near the top ofcylindrical body 12 while bottom ventilation ducts 16 are located at ornear the bottom of cylindrical body 12. Both bottom ventilation ducts 16and top ventilation ducts 15 are located around the circumference of thecylindrical body 12. The entirety of prior an VVO 2 is positioned abovegrade.

DISCLOSURE OF THE PRESENT INVENTION

It is an object of the present invention to provide a system and methodfor storing spent nuclear fuel that reduces the height of the stackassembly when a transfer cask is stacked atop a storage VVO.

It is another object of the present invention to provide a system andmethod for storing spent nuclear fuel that requires less vertical space.

Yet another object of the present invention is to provide a system andmethod for storing spent nuclear fuel that utilizes the radiationshielding properties of the subgrade during storage while providingadequate ventilation of the spent nuclear fuel.

A further object of the present invention is to provide a system andmethod for storing spent nuclear fuel that provides the same or greaterlevel of operational safeguards that are available inside a fullycertified nuclear power plant structure.

A still further object of the present invention is to provide a systemand method for storing, spent nuclear fuel that decreases the dangerspresented by earthquakes and other catastrophic events and virtuallyeliminates the potential damage from a World Trade Center or Pentagontype of attack on the stored canister.

It is also an object of the present invention to provide a system andmethod for storing spent nuclear fuel that allows an ergonomic transferof the spent nuclear fuel from a transfer cask to a storage VVO.

Another object of the present invention is to provide a system andmethod for storing spent nuclear fuel below grade.

Yet another object of the present invention is to provide a system andmethod of storing spent nuclear fuel that reduces the amount ofradiation emitted to the environment.

Still another object of the present invention is to provide a system andmethod of storing spent nuclear fuel that affords adequate heat removalcapabilities from a stored canister during flood conditions, including“smart flood” conditions.

These and other objects are met by the present invention which in oneaspect is a method of storing spent nuclear fuel comprising: providing abelow grade hole; providing a system comprising a shell forming a cavityfor receiving, a canister of spent nuclear fuel, at least a portion ofthe shell positioned below grade, and at least one inlet ventilationduct extending from an inlet to an outlet at or near a bottom of thecavity, the inlet ventilation duct connected to the shell; positioning,the apparatus in the hole so that the inlet of the inlet ventilationduct is above grade and the outlet of the inlet ventilation duct intothe cavity is below grade; filling the hole with engineered fill; andlowering; a spent fuel canister into the cavity.

In another aspect, the invention is a method of storing spent nuclearfuel comprising: providing a system comprising a structure forming acavity for receiving and storing a spent nuclear fuel canister, thecavity having a top, a bottom, and a bottom surface, at least one inletventilation duct forming a passageway from an ambient air inlet to anoutlet at or near the bottom of the cavity, and at least one outletventilation duct forming a passageway from at or near the top of thecavity to ambient air; lowering a canister loaded with spent nuclearfuel into the cavity until a bottom surface of the canister is lowerthan a top of the outlet of the at least one inlet ventilation duct; andsupporting the canister in the cavity in a position where the bottomsurface of the canister is lower than the top of the outlet of the atleast one inlet ventilation duct.

In yet another aspect, the invention is a method of storing spentnuclear fuel comprising: providing a system comprising a body having acavity for receiving and storing a spent fuel canister, a major portionof the body positioned below grade, and the body having at least oneinlet ventilation duct extending from an above grade inlet to a belowgrade outlet in the cavity; lowering the spent fuel canister into thecavity so that a major portion of the canister is below grade; andplacing a lid atop the body so as to enclose the cavity, the lid havingat least one outlet ventilation duct for allowing heated air to exit thecavity; wherein ventilation of the canister is provided by cold airentering the cavity through the inlet ventilation duct in the body, thecold air being heated within the cavity by the spent nuclear fuel, andwarm air exiting the cavity through the outlet ventilation duct in thelid.

In even yet another aspect, the invention is a method of storing spentnuclear fuel having a low level heat load comprising: providing a systemcomprising a structure forming a cavity for receiving a canister ofspent nuclear fuel, at least a portion of the cavity being positionedbelow grade, and at least one ventilation duct forming a passageway fromat or near the top of the cavity to an ambient atmosphere, wherein thecavity is hermetically sealed to ingress of below grade fluids; loweringa canister of low heat spent nuclear fuel into the cavity until at leasta major portion of the canister is below grade; and supporting thecanister in the cavity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top perspective view of a prior art VVO.

FIG. 2 is a side cross sectional view of an underground VVO according toan embodiment of the present invention having a spent fuel canisterpositioned therein.

FIG. 3 is a perspective view of the underground VVO of FIG. 2 removedfrom the ground.

FIG. 4 is a bottom perspective view of an alternate embodiment of a lidto be used with the underground VVO of FIG. 2.

FIG. 5 is a perspective view of an array of underground VVO's accordingto an embodiment of the present invention stored at an ISFSI.

FIG. 6 is a side cross sectional view of area VI-VI of FIG. 2.

FIG. 7 is a top view of the underground VVO of FIG. 2 removed from theground and with the spent fuel canister removed from the cavity and thelid removed.

FIG. 8A is a schematic cross-sectional view of an underground VVOaccording to an embodiment of the present invention having a firstalternative configuration of the inlet and outlet ventilation ducts.

FIG. 8B is a schematic cross-sectional view of an underground VVOaccording to an embodiment of the present invention having a secondalternative configuration of the inlet and outlet ventilation ducts.

FIG. 8C is a schematic cross-sectional view of an underground VVOaccording to an embodiment of the present invention having a thirdalternative configuration of the inlet and outlet ventilation ducts.

FIG. 8D is a schematic cross-sectional view of an underground VVOaccording to an embodiment of the present invention wherein the body ofthe underground VVO is substantially flush with the ground.

FIG. 8E is a schematic cross-sectional view of an underground VVOaccording to an embodiment of the present invention wherein the body ofthe underground VVO is substantially flush with the ground and having analternative configuration of the inlet and outlet ventilation ducts.

FIG. 9 is a top perspective view of an integral structure for storingspent nuclear fuel according to an embodiment of the present invention.

FIG. 10 is a schematic of the integral structure of FIG. 9 lowered intoa below grade hole and positioned atop a base.

FIG. 11 is a schematic of the arrangement of FIG. 10 wherein the belowgrade hole is being filled with soil.

FIG. 12 is a schematic illustrating the arrangement of FIG. 10 whereinthe below grade hole is completely filled with soil.

FIG. 13 is a schematic illustrating the arrangement of FIG. 12 wherein aspent fuel canister is loaded in the integral structure and a lidpositioned thereon.

FIG. 14 is a schematic view of an integral structure according to anembodiment of the present invention having an alternative configurationor the inlet and outlet ventilation ducts.

FIG. 15 is a schematic view of an integral structure for storing lowheat spent fuel according to an embodiment of the present invention freeof inlet ventilation ducts.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring to FIGS. 2 and 3, underground VVO 20 is illustrated accordingto a first embodiment of the present invention. Underground VVO 20 is avertical, ventilated dry spent fuel storage system that is fullycompatible with 100 ton and 125 ton transfer casks for spent fuelcanister transfer operations. Underground VVO 20 can bemodified/designed to be compatible with any size or style transfer cask.Underground VVO 20 is designed to accept spent fuel canisters forstorage at an Independent Spent Fuel Storage Installation “ISFSI”) inlieu of above ground overpacks (such as prior art VVO 2 in FIG. 1). Allspent fuel canister types engineered for storage in free-standing andanchored overpack models can be stored in underground VVO 20.

As used herein the term “canister” broadly includes any spent fuelcontainment apparatus, including, without limitation, multi-purposecanisters and thermally conductive casks. For example, in some areas ofthe world, spent fuel is transferred and stored in metal casks having ahoneycomb grid-work/basket built directly into the metal cask. Suchcasks and similar containment apparatus qualify as canisters, as thatterm is used herein, and can be used in conjunction with underground VVO20 as discussed below.

Underground VVO 20 comprises body 21, base 22, and removable lid 41.Body 21 is constructed of concrete, but can be constructed of othersuitable materials. Body 21 is rectangular in shape but can be anyshape, such as for example, cylindrical, conical, spherical,semi-spherical, triangular, or irregular in shape. A portion of body 21is positioned below grade so that only top portion 24 protrudes abovegrade level 23. Preferably, at least a major portion of the height ofbody 21 is positioned below grade. The exact height which top portion 24of body 21 extends above ground level 23 can be varied greatly and willdepend on a multitude of design considerations, such as canisterdimensions, radioactivity levels of the spent fuel to be stored, ISFSIspace limitations, geographic location considering susceptibility tomissile-type and ground attacks, geographic location consideringfrequency of and susceptibility to natural disasters (such asearthquakes, floods, tornadoes, hurricanes, tsunamis, etc.),environmental conditions (such as temperature, precipitation levels),and/or ground water levels. Preferably, top portion 24 of body 21 isless than approximately 42 inches above ground level 23, and mostpreferably approximately 6 to 36 inches above ground level 23.

In some embodiments, it may even be preferable that the entire height ofbody 21 be below grade (illustrated in FIGS. 8D and 8E). As will bediscussed in more detail below, when the entire height of body is belowgrade, only the top surface of the body will be exposed to the ambientair above grade.

Referring still to FIGS. 2 and 3, body 21 forms cylindrical cavity 26therein (best shown in FIG. 3). While cavity 26 is cylindrical in shape,cavity 26 is not limited to any specific size, shape, and/or depth andcan be designed to receive and store almost any shape of canisterwithout departing from the spirit of the invention. While not necessaryto practice the invention, it is preferred that the horizontalcross-sectional size and shape of cavity 26 be designed to generallycorrespond to the horizontal cross-sectional size and shape of thecanister-type that is to be used in conjunction with that particularunderground VVO. More specifically, it is desirable that the size andshape of cavity be designed so that when a spent fuel canister (such ascanister 70) is positioned in cavity 26 for storage, a small clearanceexists between the outer side walls of the canister and the side wallsof cavity 26.

Designing cavity 26 so that a small clearance is formed between the sidewalls of the stored canister and the side walls of cavity 26 limits thedegree the canister can move within the cavity during a catastrophicevent, thereby minimizing damage to the canister and the cavity wallsand prohibiting the canister from tipping over within the cavity. Thissmall clearance also facilitates flow of the heated air during spentnuclear fuel cooling. The exact size of the clearance can becontrolled/designed to achieve the desired fluid flow dynamics and beattransfer capabilities for any given situation. In some embodiments, forexample, the clearance may be 1 to 3 inches. A small clearance alsoreduces radiation streaming.

Two inlet ventilation ducts 25 are provided in body 21 for providinginlet ventilation to the bottom of cavity 26. Inlet ventilation ducts 25are elongated substantially S-shaped passageways extending from abovegrade inlets 27 to below grade outlets 28. Above grade inlets 27 arelocated on opposing side walls of top portion 24 of body 21 and open tothe ambient air above ground level 23. As use herein, the terms ambientair, ambient atmosphere, or outside atmosphere, refer to theatmosphere/air external, to the underground VVO, and include the naturaloutside environment and spaces within buildings, tents, caves, tunnels,or other man-made or natural enclosures.

Below grade outlets 28 open into cavity 26 at or near its bottom at aposition below the ground level 23. Thus, inlet ventilation ducts 25provide a passageway for the inlet of ambient air to the bottom ofcavity 26, despite the bottom of cavity 26 being well below grade. Ventscreens 31 (FIG. 3) are provided to cover above grade inlets 27 so thatobjects and other debris can not enter and block the passageways ofinlet ventilation ducts 25. As a result of the elongated S-shape ofinlet ventilation ducts 25, above grade inlets 27 cease to be a locationof elevated dose rate that is common in free-standing above ground VVOs.While below grade outlets 28 are illustrated as being opening near thebottom of the walls of cavity 26, below grade outlets 28 can be locatedin the floor of cavity 26 is desired. This can be accomplished byappropriately reshaping inlet ventilation ducts 25 and forming anopening through bottom plate 38 and into cavity 26. In such anembodiment, base 22 can be considered part of the body 21 through whichthe inlet ventilation ducts 25 extend.

Above grade inlets 27 are located in the side walls of body 21 at anelevation of about 10 inches above ground level 23. However, theelevation of above grade inlets 27 is not limiting of the presentinvention. The inlets 27 can be located at any desired elevation abovethe ground level, including level/flush therewith, as shown in FIGS. 8Dand 8E. Elevating above grade inlets 27 substantially above the groundlevel 23 helps reduce the likelihood that rain or flood water will enterthe cavity 26. It is noted that for IFSI's in flood zones, floodwatercan possibly rise more than a foot above ground level and, thus, entercavity 26 via inlet ventilation ducts 25. However, as discussed belowwith respect to FIG. 6, underground VVO 20 is specifically designed todeal with the worst flood conditions in a safe and effective manner.

While above grade inlets 27 are preferably located in the side walls ofbody 21, the above grade inlets are not limited to such a location and,if desired, can be located anywhere on the body, including for examplein the top surface (or any other surface) of the body. Further examplesof possible locations for above grade inlets 27 on body 21 areillustrated in FIGS. 8A-8E.

Referring still to FIGS. 2 and 3, inlet ventilation ducts 25 have arectangular cross-sectional area of about 6 inches by 40 inches.However, any cross-sectional shape and/or size can be used, such as forexample, round, elliptical, triangular, hexagonal, octagonal, etc.Additionally, while the shape of inlet ventilation ducts 25 is anelongated substantially S-shaped passageway, a multitude of shapes canbe used that still achieve acceptable dose rates at the above gradeinlets 27. For example, rather than an elongated S-shape, the inletventilation duct can extend from the above grade inlet to the belowgrade outlet in a zig-zag shape, a tilted linear shape, a generalL-shape, or any angular, linear, or curved combination. The exact shape,size, and cross-sectional configuration of the inlet ventilation duct isa matter of design preference and will be dictated by such factors, suchas thickness of the body of the VVO, radioactivity level of the spentfuel being stored in the cavity, temperature of the spent fuel canister,desired fluid flow dynamics through the ducts, and placement of theabove grade inlet vents on the body (i.e., whether the above grade inletvents/opening are located on the side walls of the body, its topsurface, or some other surface of the body). Further examples ofpossible shapes for inlet ventilation ducts 25 are illustrated in FIGS.8A-8E.

Inlet ventilation ducts 25 are preferably formed by a low carbon steelliner. However, inlet ventilation ducts 25 can be made of any materialor can be mere passageways formed into concrete body 21 without alining.

As best illustrated in FIG. 3, cavity 26 is formed by thick steel shell34 and bottom plate 38. Shell 34, bottom plate 38, and inlet ventilationducts 25 are preferably made of a metal, such as steel, preferably lowcarbon steel, but can be made of other materials, such as stainlesssteel, aluminum, aluminum-alloys, plastics, and the like. Inletventilation ducts 25 are seal joined to shell 34 and bottom plate 38 toform an integral/unitary structure 100 (shown in isolation in FIG. 9)that is hermetically sealed to the ingress of below grade water andother fluids. In the case of weldable metals, this seal joining maycomprise welding or the use of gaskets. Thus, the only way water orother fluids can enter cavity 26 is through above grade inlets 27 oroutlet ventilation ducts 42 in lid 41. As will be discussed below withrespect to FIGS. 9-15, the integral structure itself is an invention andcan be used to store spent nuclear fuel without the use of body 21.

An appropriate preservative, such as a coal tar epoxy or the like, isapplied to the exposed surfaces of shell 34, bottom plate 38, and inletventilation ducts 25 in order to ensure sealing, to decrease decay ofthe materials, and to protect against fire. A suitable coal tar epoxy isproduced by Carboline Company out of St. Louis, Mo. under the tradenameBitumastic 300M. In some embodiments of the underground VVO of thepresent invention, a bottom plate will not be used.

Concrete body 21 surrounds shell 34 and inlet, ventilation ducts 25.Body 21 provides non-structural protection for shell 34 and inletventilation ducts 25. Insulation 37 is provided at the interface betweenshell 34 and concrete body 21 and at the interface between inletventilation ducts 25 and concrete body 21. Insulation 37 is provided toprevent excessive transmission of heat decay from spent fuel canister 70to concrete body 21, thus maintaining the bulk temperature of theconcrete within FSAR limits. Insulating shell 34 and inlet ventilationducts 25 from concrete body 21 also serves to minimize the heat-up ofthe incoming cooling air before it enters cavity 26. Suitable forms ofinsulation include, without limitation, blankets of alumina-silica fireclay (Kaowool Blanket), oxides of alimuna and silica (Kaowool SBlanket), alumina-silica-zirconia fiber (Cerablanket), andalumina-silica-chromia (Cerachrome Blanket).

Insulating, inlet ventilation ducts 25 from the heat load of spent fuelin cavity 26 is very important in facilitating and maintaining adequateventilation/cooling of the spent fuel. The insulating process can beachieved in a variety of ways, none of which are limiting of the presentinvention. For example, in addition to adding, an insulating material tothe exterior of the shell 34 and inlet ventilation ducts 25, it is alsopossible to insulate inlet ventilation ducts 25 by providing a gap inconcrete body 21 between cavity 26 and inlet ventilation ducts 25. Thegap may be filled with an inert gas or air if desired. Moreover,irrespective of the means used to provide the insulating effect, theinsulating means is not limited to being positioned on the outsidesurfaces of shell 34 or inlet ventilation ducts 25 but can be positionedanywhere between cavity 26 and inlet ventilation ducts 25.

Body 21, along with the integral steel unit formed by bottom plate 38,shell 34, and ventilation ducts 25, are placed atop base 22. Base 22 isa reinforced concrete slab designed to satisfy the load combinations ofrecognized industry standards, such as, without limitation, ACI-349.Base 22 is rectangular in shape but can take on any shape necessary tosupport body 21, such as round, elliptical, triangular, hexagonal,octagonal, irregularly shaped, etc. While using a base is preferable toachieve adequate load supporting requirements, situations can arisewhere using such a base may be unnecessary.

Referring back to FIG. 2, underground VVO 20 has a removable ventilatedlid 41. Lid 41 is positioned atop body 21, thereby substantiallyenclosing cavity 26 so that radiation does not escape through the top ofcavity 26 when canister 70 is positioned in cavity 26. When lid 41 isplaced atop body 21 and spent fuel canister 70 is positioned in cavity26, outlet air plenum 36 is formed between the top surface of canister70 and lid 41. Outlet air plenum 36 is preferably a minimum of 3 inchesin height, but can be any desired height. The exact height will bedictated by design considerations such as desired fluid flow dynamics,canister height, VVO height, the depth of the cavity, canister heatload, etc.

Lid 41 has four outlet ventilation ducts 42. Outlet ventilation ducts 42form a passageway from the top of cavity 26 (specifically from outletair plenum 36) to the ambient air so that heated air can escape fromcavity 26. Outlet ventilation ducts 42 are horizontal passageways thatextend through side wall 30 of lid 41. However, the outlet ventilationducts can be any shape or orientation, such as vertical, L-shaped,S-shaped, angular, curved, etc. Because outlet ventilation ducts 42 arelocated within lid 41 itself, the total height of body 21 is minimized.

Lid 41 comprises a roof 35 made of concrete. Roof 35 provides radiationshielding so that radiation does not escape from the top of cavity 26.Side wall 30 of lid 41 is an annular ring. Outlet air plenum 36 helpsfacilitate the removal of heated air via outlet ventilation ducts 42. Inorder to minimize the heated air exiting outlet ventilation ducts 42from being siphoned back into inlet ventilation ducts 25, outletventilation ducts 42 are azimuthally and circumferentially separatedfrom inlet ventilation ducts 25.

Ventilated lid 41 also comprises shear ring 47. When lid 41 is placedatop body 21, shear ring 47 protrudes into cavity 26, thus, providingenormous shear resistance against lateral forces from earthquakes,impactive missiles, or other projectiles. Lid 41 is secured to body 21with bolts (not shown) that extend therethrough.

While not illustrated, it is preferable that duct photon attenuators beinserted into all of inlet ventilation ducts 25 and/or outletventilation ducts 42 of underground VVO 20, irrespective of shape and/orsize. A suitable duct photon attenuator is described in U.S. Pat. No.6,519,307, Bongrazio, the teachings of which are incorporated herein byreference.

Referring now to FIG. 4, an embodiment of a lid 50 that can be used inunderground VVO 20 is illustrated. Lid 50 contains similar designaspects as lid 41 and is illustrated to more fully disclose theaforementioned lid design aspects. Lid 50 has four horizontal outletventilation ducts 51 in side wall 52. Shear ring 54 is provided on thebottom of lid 50 to fit into cavity 26. Bolts 18 are used to secure lid50 to tapped holes in the top of body 21.

While the outlet ventilation ducts are illustrated as being locatedwithin the lid 50 of underground VVO 20, the present invention is not solimited. For example, outlet ventilation ducts can be located in thebody of the underground VVO at a location above grade. This concept isillustrated if FIGS. 8A-8E. If the outlet ventilation ducts are locatedin the body of the underground VVO, the openings of the outletventilation ducts to the ambient air can be located in the body's sidewalls, on its top surface, or in an other surface. Similar to when theoutlet ventilation ducts are located in the lid, the outlet ventilationducts can take on a variety of shapes and/or configurations when locatedin the body of the underground VVO itself. As with the inlet ventilationducts, the outlet ventilation ducts are preferably formed by a lowcarbon steel liner, but can be made of any material or can be merepassageways formed into concrete body 21 or lid 41 without a lining. Inall embodiments of the present invention which have both inlet andoutlet ventilation ducts, it is preferred that the outlet ventilationduct openings be azimuthally and circumferentially separated, from theinlets of the inlet ventilation ducts to minimize interaction betweeninlet and outlet air streams. There is no limitation on the shape andstyle of lid used in conjunction with underground VVO 20.

Referring back to FIG. 2, soil 29 surrounds body 21 for almost theentirety of its height. When spent fuel canister 70 is positioned incavity 26, at least a major portion, if not the entirety, of canister 79is below grade. Preferably, the entire height of canister 70 is belowgrade in order to take full advantage of the shielding, effect of thesoil 29. Thus, soil 29 provides a degree of radiation shielding forspent fuel stored in underground VVO 20 that can not be achieved inabove-ground overpacks. Underground VVO 20 is unobtrusive in appearanceand there is no danger of underground VVO 20 tipping over. Additionally,underground VVO 20 does not have to contend with soil-structureinteraction effects that magnify the free-field acceleration andpotentially challenge the stability of an above ground free-standingoverpack.

Referring to FIG. 6, area VI-VI of FIG. 2 is illustrated in detail. FIG.6 illustrates design aspects that are important to ensure thatunderground VVO 20 can successfully withstand flood conditions withoutadverse impact. Support blocks 32 are provided on the bottom surface(formed by plate 38) of cavity 26 so that canister 70 can be placedthereon. Support blocks 32 are circumferentially spaced from one another(shown in FIG. 7). When canister 70 is loaded into cavity 26 forstorage, the bottom surface 71 of canister 70 rests on support blocks32, forming an inlet air plenum 33 between the bottom surface 71 of thecanister 70 and the bottom surface/floor of cavity 26. Support blocks 32are made of low carbon steel and are preferably welded to the bottomsurface of the cavity 26. Other suitable materials of constructioninclude, without limitation, reinforced-concrete, stainless steel, andother metal alloys.

Support blocks 32 also serve an energy/impact absorbing function.Support blocks 32 are preferably of a honeycomb grid style, such asthose manufactured by Hexcel Corp., out of California, U.S.

Support blocks 32 are specifically designed so that bottom surface 71 ofcanister 70 is lower than top 74 of below grade outlets 28 (FIG. 2) ofinlet ventilation ducts 25. Preferably, support blocks 32 are designedso that bottom surface 71 of canister 70 is about 2 to 6 inches belowtop 74 of below grade outlets 28. However, any desired heightdifferential can be achieved through proper design. By supportingcanister 70 in cavity 26 so that its bottom surface 71 is lower than top74 of below grade outlets 28, underground VVO 20 will provide adequatecooling to canister 70 under even the most adverse flood condition,which is colloquially referred to as a “smart flood.” A “smart flood” isone that floods the VVO so that the water level is just high enough toblock airflow though the inlet ventilation ducts 25 completely. In otherwords, the water level is just even with top 74 of the below gradeoutlets 28.

However, underground VVO 20 can adequately deal with the “smart flood”condition because the bottom surface 71 of the canister 70 is situatedat a height that is below top 74 of below grade outlets 28. As a result,if a “smart flood” was to occur, the bottom of the canister 70 will bein contact with (i.e. submerged in) the water. Because the heat removalefficacy of water is over 100 times that of air, a wet bottom is allthat is needed to effectively remove heat and keep the canister 70 cool.The deeper the submergence of canister 70 in the water, the coolercanister 70 and its contained fuel will remain. As the water in cavity26 is heated by the bottom of canister 70, the water evaporates, risesthrough cavity 26 via annular space 60, and exits cavity 26 via theoutlet ventilation ducts. Thus, the canister cooling action changes fromventilation air-cooling to evaporative water cooling.

In one embodiment, below grade outlets 28 of inlet ventilation ducts 25will be 8 inches high by 40 inches wide and inlet air plenum 33 is 6inches high. This provides a height differential of 2 inches.

It should be noted that the height differential design aspect ofunderground VVO 20 that is detailed in FIG. 6 can also be incorporatedinto free-standing above ground casks and VVOs to deal with “smartflood” conditions, independent of the other features of underground VVO20. Thus, this concept is an independent inventive aspect of the presentapplication. When incorporated into above ground VVOs, the inletventilation duets should be designed so that radiation can not escape tothe surrounding environment front the inlet ventilation ducts. This is athreat because the canister will be below the inlet duct's opening, intothe storage cavity. In this embodiment, the inlet ventilation ducts willbe shaped so that a line of sight does not exist to the canister in thestorage cavity from the ambient air. For example, the inlet ventilationducts can comprise a portion that is L-shaped, angled, S-shaped, orcurved.

Moreover, while the height differential design aspect of FIG. 6 isachieved using support blocks 32, it is also possible to practice thisaspect of the invention without support blocks 32. In such embodiments,canister 70 will be positioned in cavity 26 and rest directly on thefloor of cavity 26. However, the use of support blocks 32 is desirablebecause of the creation of air inlet plenum 33 and because the use ofsupport blocks 32 helps prohibit debris and dirt from getting trapped atthe bottom of cavity 26.

Referring now to FIGS. 8A-8E, examples of alternative configurations ofthe outlet ventilation ducts and the inlet ventilation ducts in anunderground VVO according to the present invention are schematicallyillustrated. Much of the detail, and some structure, has been omitted inFIGS. 8A-8E for simplicity with the understanding that any or all of thedetails discussed above with respect to underground VVO 20 can beincorporated therein. Like numbers are used to identify like parts withthe exception of alphabetical suffixes being used for each embodiment.

It should be noted that in addition to the configurations of the inletventilation ducts and the outlet ventilation ducts illustrated in FIGS.8A-8E, a multitude of other configurations, combinations, andmodifications can be incorporated into the present invention. Some ofthese details are discussed above. Additionally, the outlet ventilationduct configurations of any of the illustrated embodiments can becombined with any of the illustrated inlet ventilation ductconfigurations, and vice versa.

In all embodiments of the present invention, it is desirable that theheated air exiting the outlet ventilation ducts 42 be prohibited frombeing siphoned back into the inlet ventilation ducts 25 (i.e., keepingthe warm outlet air stream from mixing with the cool inlet air stream).This can be accomplished by in a number of ways, including: (1) thepositioning/placement of the inlets 27 on the underground VVO 20 withrespect to the outlets of the outlet ventilation ducts 42; providing aplate 98 or other structure that segregates the air streams (asexemplified in FIGS. 8A and 8C-8E); and/or (3) extending the inletventilation ducts 25 to a position away from the outlet ventilationducts 42.

As a result of the heat emanating from canister 70, cool air from theambient is siphoned into inlet ventilation ducts 25 and into the bottomof cavity 26. This cool air is then warmed by the heat from the spentfuel in canister 70, rises in cavity 26 via annular space 60 (FIG. 6)around canister 70, and then exits cavity 26 as heated air via outletventilation ducts 42 in lid 41.

Referring now to FIG. 5, ISFIs can be designed to employ any number ofunderground VVOs 20 for integral structures 100) and can be expanded innumber easily to meet growing needs. Although underground VVOs 20 areclosely spaced, the design permits any cavity to be independentlyaccessed by cask crawler 90 with ease. The subterranean configuration ofunderground VVOs 20 greatly reduce the height of the stack structurescreated during loading/transfer procedures where transfer cask 80 ispositioned atop underground VVO 20.

An embodiment of a method of using underground VVO 20 to store spentnuclear fuel canister 70 will now be discussed in relation to FIGS. 2-5.Upon being removed from a spent fuel pool and treated for dry storage,spent fuel canister 70 is positioned in transfer cask 80. Transfer caskis 80 is carried by cask crawler 90 to a desired underground VVO 20 forstorage. While a cask crawler is illustrated, any suitable means oftransporting transfer cask 80 to a position above underground VVO 20 canbe used. For example, any suitable type of load-handling device, such aswithout limitation, a gantry crane, overhead crane, or other cranedevice can be used.

In preparing the desired underground VVO 20 to receive canister 70, lid41 is removed from body 21 so that cavity 26 is open. Cask crawler 90positions transfer cask 80 atop underground VVO 20. After transfer caskis properly secured to the top of underground VVO 20, a bottom plate oftransfer cask 80 is removed. If necessary, a suitable mating device canbe used to secure the connection of transfer cask 80 to underground VVO20 and to remove the bottom plate of transfer cask 80 to an unobtrusiveposition. Such mating, devices are well known in the art and are oftenused in canister transfer procedures. Canister 70 is then lowered bycask crawler 90 from transfer cask 80 into cavity 26 of underground VVO20 until the bottom surface of canister 70 contacts and rests atopsupport blocks 32, as described above.

When resting on support blocks 32, a major portion of the canister'sheight is below grade. Most preferably, the entirety of canister 70 isbelow grade when in its storage position. Once canister 70 is positionedand resting in cavity 26, lid 41 is placed over cavity 26, substantiallyenclosing cavity 26. Lid 41 is oriented atop body 21 so that shear ring47 protrudes into cavity 26 and outlet ventilation ducts 42 areazimuthally and circumferentially separated from inlet ventilation ducts25 on body 21. Lid 41 is then secured to body 21 with bolts. As a resultof the heat emanating from canister 70, cool air from the ambient issiphoned into inlet ventilation ducts 25 and into the bottom of cavity26. This cool air is then warmed by the heat from the spent fuel incanister 70, rises in cavity 26 via annular space 60 (FIG. 6) aroundcanister 70, and then exits cavity 26 as heated air via outletventilation ducts 42 in lid 41.

Referring now to FIG. 9, an integral structure 100 for storing spentnuclear fuel is illustrated according to an embodiment of the invention.Integral structure 100 is essentially a combination of shell 34, inletventilation ducts 25, and bottom plate 38 of underground VVO 20 withoutthe concrete body. Integral shell 100 can be used to store canisters ofspent nuclear fuel without the addition of the concrete body. Therefore,some embodiments of the present invention will be the integral structure100 itself.

Shell 34, bottom plate 38, and inlet ventilation ducts 25 are preferablyformed of a metal, such as low carbon steel. Other suitable materialsinclude, without limitation, stainless steel, aluminum, aluminum-alloys,plastics, and the like.

Inlet ventilation ducts 25, bottom plate 38, and shell 34 are sealwelded at all junctures to form a unitary structure that is hermeticallysealed to the ingress water and other fluids. The only way water orother fluids can enter cavity 26 is through inlets 27 or top opening 101of shell 34. The height of shell 34 is designed so that a canister ofspent fuel can be positioned within cavity 26 so as not to protrude fromtop opening 101. There is no limitation on the height to which shell 34can be constructed. The exact height of shell 34 will be dictated by theheight of the spent fuel canister to be stored therein, the desireddepth (below grade) at which the canister is to be stored, whether theoutlet ventilation ducts are in the lid or integrated into the shell 34,and/or the desired height of the outlet air plenum that is to existduring canister storage.

FIGS. 10-13 illustrate a process of using integral structure 100 tostore a spent fuel canister at a below grade position at an ISFSI, orother location, according to one embodiment of the present invention. Itshould be noted that the any of the design and/or structural detailsdiscussed above with respect to underground VVO 20 can be incorporatedinto integral structure 100, such as, for example, the use of ventscreens, variable configurations of the inlet and outlet ducts,clearances, the use of an insulation, etc. However, in order to avoidredundancy, a discussion of these details will be omitted with theunderstanding that any or all of the details of underground VVO 20 are(or can be) incorporated into the storing methods and apparatus ofintegral structure 100, and vice versa.

Referring to FIG. 10, a hole 200 is first dug, into the ground 210 at adesired position within the ISFSI and at a desired depth. Once hole 200is dug, and its bottom properly leveled, base 22 is placed at the bottomof hole 200. Base 22 is a reinforced concrete slab designed to satisfythe load combinations of recognized industry standards, such as ACI-349.However, in some embodiments, depending on the load to be supportedand/or the ground characteristics, the use of a base may be unnecessary.

Once base 22 is properly positioned in hole 200, integral structure 100is lowered into the hole 200 in a vertical orientation until it restsatop base 22. Bottom plate 38 of integral structure 100 contacts andrests atop the top surface of base 22. If desired, the bottom plate 38can be bolted or otherwise secured to the base 22 at this point toprohibit future movement of the integral structure 100 with respect tothe base 22.

Referring to FIG. 11, once integral structure 100 is resting atop base22 in the vertical orientation, soil supply pipe 300 is moved intoposition above hole 200. Soil 301 is delivered into hole 200 exterior ofintegral structure 100, thereby filling hole 200 with soil 301 andburying, a portion, of the integral structure 100. While soil 301 isexemplified to fill hole 200, any suitable engineered fill can be usedthat meets environmental and shielding requirements. Other suitableengineered fills include, without limitation, gravel, crushed rock,concrete, sand, and the like. Moreover, the desired engineered fill canbe supplied to the hole by any means feasible, including manually,dumping, and the like.

Referring to FIG. 12, soil 301 is supplied to hole 200 until soil 301surrounds integral structure 100 and fills hole 200 to a level wheresoil 301 is approximately equal to ground level 212. Soil 301 is indirect contact with the exterior surfaces of integral structure 100 thatare below grade. When hole 200 is filled with soil 301, inlets 27 ofinlet ventilation ducts 25 are above grade. Shell 34 also protrudes fromsoil 301 so that opening 101 is slightly above grade. Therefore, becauseintegral structure 100 is hermetically sealed at all junctures, belowgrade liquids and soil can not enter into cavity 26 or inlet ventilationducts 25. Support blocks 32 are provided at the bottom of cavity 26 forsupporting a stored spent fuel canister.

Referring to FIG. 13, once hole 200 is adequately filled with soil 301,a canister 70 of spent fuel 70 is loaded into cavity 26 of integralstructure 100. The canister loading sequence is discussed in greaterdetail above with respect to FIG. 5. Canister 70 is lowered into cavity26 until it rests on support blocks 32. As discussed above with respectto FIG. 6, support blocks 32 and outlets 28 of integral structure 100are specially designed to deal with “smart flood” conditions. Canister70 rests on support blocks 32, forming an inlet air plenum 33 betweenthe bottom of canister 70 and the floor of cavity 26 (which in this caseis bottom plate 38).

When canister 70 is supported on support blocks 32, the entire height ofcanister 70 is below ground level 212. This maximizes use of theground's radiation shielding capabilities. The depth at which canister70 is below ground level 212 can be varied by increasing or decreasingthe depth of hole 200. Once canister 70 is supported in cavity 26, lid41 is placed atop shell 34, thereby closing opening 101 and prohibitingradiation from escaping upwards from cavity 26. Outlet air plenum 36 isformed between the bottom surface of lid 41 and the top of canister 70.

Lid 41 comprises outlet ventilation ducts 42. Outlet ventilation ducts42 form passageways from outlet air plenum 36, through lid 41, to theambient air above ground level 212. Outlet ventilation ducts 42 do nothave to be provided in lid 41, but can be formed as part of the integralstructure 100 if desired. This will be discussed in greater detail belowwith respect to FIG. 14.

Referring still to FIG. 13, when integral structure 100 is used to storespent nuclear fuel canister 70, the radiation shielding effect of thesub-grade is utilized while adequately facilitating cooling of canister70. The cooling of canister 70 is facilitated by cool air entering inletventilation ducts 25 via above grade inlets 27. The cool air travelsthrough inlet ventilation ducts 25 until it enters cavity 26 at or nearinlet air plenum 33 via below grade outlets 28. Once the cool air iswithin cavity 26 it is warmed by the heat emanating from canister 70. Asthe air is warmed, it travels upward along the outer surface of canister70 via annular space 60 until the air enters outlet air plenum 36. Asthe air travels upward through annular space 60 it continues to removeheat from canister 70. The warmed air then exits cavity 26 via outletventilation ducts 42 and enters the ambient air. This natural convectivecooling flow repeats continuously until the canister 70 is adequatelycooled.

Referring now to FIG. 14, an alternative embodiment of an integralstructure 200 is illustrated. Integral structure 200 is used to store aspent fuel canister in manner similar to that of integral structure 100discussed above. While much of the structure is identical to that ofintegral structure 100, integral structure 200 further comprises outletventilation ducts 42 seal welded directly to shell 34. The outletventilation ducts 42 can be formed out of any of the materials discussedabove with respect to the inlet ventilation ducts 25. As a result of theoutlet ventilation ducts 42 being part of integral structure 200, lid 41can be free of such ducts. The cooling process of canister 70 remainsthe same.

FIG. 15 illustrates an integral structure 300 according to anotheraspect of the present invention. Integral structure 300 is similar inmany respect to that of integral structures 100 and 200 in its designand functioning. However, integral structure 300 is specificallydesigned to store canisters 70 holding low heat spent fuel. When acanister 70 is giving off low heat, for example in the magnitude of 2-3kW, it is not necessary to supply inlet ventilation ducts to supply coolair to cavity 26. Therefore, the inlet ventilation ducts are omittedfrom integral structure 300. Integral structure 300 comprises onlyoutlet ventilation ducts 42, which act as both an inlet for the coolerair and an outlet for the warmer air.

While outlet ventilation ducts 42 of integral structure 300 are sealwelded to shell 34, it is possible for the outlet ventilation ducts tobe located in the lid 41 if desired. Moreover, the concept ofeliminating the inlet ventilation ducts for low heat load canisterstorage can be applied to any of the underground or above ground VVOembodiments illustrated in this application, specifically includingunderground VVO 20 and it derivatives.

While the invention has been described and illustrated in sufficientdetail that those skilled in this art can readily make and use it,various alternatives, modifications, and improvements should becomereadily apparent without departing from the spirit and scope of theinvention. Specifically, it is possible for the entire underground VVOand/or integral structure of the present invention to be below grade, solong as the inlet ventilation ducts and/or outlet ventilation ducts opento the ambient air above grade. This facilitates very deep storage ofspent fuel canisters.

What is claimed is:
 1. A unitary storage structure for storing spentnuclear fuel, the structure comprising: a metal shell forming a cavityconfigured to store a spent nuclear fuel canister, the shell having anopen top end, a closed metal bottom plate seal welded to a bottom end ofthe shell to prevent the ingress of water and other fluids, a height,and an opening in a side wall of the shell; an elongated metal inletventilation duct disposed externally to the shell, the inlet ventilationduct having an inlet and an outlet in fluid communication with thecavity of the canister, the outlet being seal welded to the opening ofthe shell forming a sealed passageway from the inlet to the opening ofthe shell; the outlet of the inlet ventilation duct being positioned ata first vertical height above the bottom end of the shell and the inletbeing positioned at a second vertical height above the bottom end of theshell, wherein the second vertical height is greater than the firstvertical height; wherein the metal bottom plate has an extension portionthat extends outwards beyond the shell, the inlet ventilation duct beingseal welded to a top of the extension portion to prevent the ingress ofwater and other fluids; wherein the shell, metal bottom plate, and inletventilation duct form a self-supporting unitary storage structure. 2.The unitary storage structure of claim 1, wherein the inlet ventilationduct includes a vertical section that is spaced apart from the shell. 3.The unitary storage structure of claim 2, wherein the inlet ventilationduct includes a horizontal section fluidly coupled to the verticalsection and to the opening of the shell.
 4. The unitary storagestructure of claim 1, wherein the second vertical height of the inlet ofthe inlet ventilation duct is higher than the top end of the shell. 5.The unitary storage structure of claim 1, wherein the shell has acylindrical shape.
 6. The unitary storage structure of claim 1, furthercomprising: a lid positioned atop the shell so as to substantiallyenclose the open top end of the shell, the lid being non-unitary withrespect to the shell; the lid comprising an outlet ventilation ductforming a passageway from a top of the cavity to ambient atmosphere. 7.The unitary storage structure of claim 1, further comprising an outletventilation duct forming a passageway from a top of the cavity toambient atmosphere.
 8. A unitary storage structure for storing spentnuclear fuel, the structure comprising: a metal shell forming a cavityconfigured to store a spent nuclear fuel canister, the shell having anopen top end, a closed metal bottom plate seal welded to a bottom end ofthe shell, a height, and an opening in a side wall of the shell; anelongated metal inlet ventilation duct disposed externally to the shell,the inlet ventilation duct having an inlet in fluid communication withambient atmosphere and an outlet in fluid communication with the cavityof the canister, the outlet being seal welded to the opening of theshell forming a sealed passageway from the inlet to the opening of theshell; the outlet of the inlet ventilation duct being positioned at afirst vertical height above the bottom end of the shell and the inletbeing positioned at a second vertical height above the bottom end of theshell, wherein the second vertical height is greater than the firstvertical height; wherein the inlet ventilation duct is seal welded tothe bottom plate of the shell, the shell, bottom plate, and inletventilation duct forming a self-supporting unitary storage structure. 9.The unitary storage structure of claim 8, wherein the inlet ventilationduct is welded to the opening of the shell and bottom plate of theshell.
 10. The unitary storage structure of claim 8, wherein the bottomplate has an extension portion that extends outwards beyond the shell,the inlet ventilation duct being welded to the extension portion. 11.The unitary storage structure of claim 8, wherein the inlet ventilationduct includes a vertical section that is spaced apart from the shell.12. The unitary storage structure of claim 8, wherein the inletventilation duct includes a horizontal section fluidly coupled to thevertical second and to the opening of the shell.
 13. A unitary storagestructure for storing spent nuclear fuel, the structure comprising: ametal shell forming a cavity configured to store a spent nuclear fuelcanister, the shell having an open top end, a closed metal bottom plateseal welded to a bottom end of the shell, a height, and an opening in aside wall of the shell; an elongated metal inlet ventilation ductdisposed externally to the shell, the inlet ventilation duct having aninlet in fluid communication with ambient atmosphere and an outlet influid communication with the cavity of the canister, the outlet beingseal welded to the opening of the shell forming a sealed passageway fromthe inlet to the opening of the shell; wherein the inlet ventilationduct includes a vertical section that is spaced apart from the shell anda horizontal section fluidly coupled to the vertical section and to theopening of the shell; wherein the horizontal section of the inletventilation duct is seal welded to the bottom plate of the shell and theshell forming a self-supporting unitary storage structure comprised ofthe inlet ventilation duct, bottom plate, and shell.
 14. The unitarystorage structure of claim 13, wherein the outlet of the inletventilation duct being positioned at a first vertical height above thebottom end of the shell and the inlet being positioned at a secondvertical height above the bottom end of the shell, wherein the secondvertical height is greater than the first vertical height.
 15. Theunitary storage structure of claim 13, wherein the inlet ventilationduct is welded to the opening of the shell and bottom plate of theshell.
 16. The unitary storage structure of claim 13, wherein the bottomplate has an extension portion that extends outwards beyond the shell,the inlet ventilation duct being welded to the extension portion.